This invention relates to a method for inspecting a nuclear fuel rod or irradiation capsule.
Uranium carbide, uranium nitride, plutonium carbide or plutonium nitride or mixture compounds thereof are desirable nuclear fuels having high power densities (especially, for use in fat neutron reactors) because these materials have high melting points and high thermal conductivities. Various attempts have been made to effectively transfer energy radiated from any one of these nuclear fuel materials of high power densities to a coolant. In one of the prior art attempts, a heat transfer medium having a high thermal conductivity such as sodium or NaK is charged into a gap defined between the nuclear fuel material and a sleeve enclosing the fuel material to provide a thermal conductivity higher than that obtainable when gas (such as helium) is charged as the heat transfer medium in the gap, to thereby further increase the power density of the whole core of the reactor.
This technology can be also equally applied to an irradiation capsule which is employed in an irradiation test of an experimental nuclear fuel rod for experiment in a laboratory. The irradiation capsule generally comprises a plurality of concentric and spaced tubes with the innermost tube directly enclosing the nuclear fuel rod and the succeeding outer tubes surrounding their adjacent inner tubes in succession and a heat transfer medium such as Na or He charged into gaps defined between the fuel rod and innermost tube and between the adjacent tubes. The thus prepared irradiation capsule is inserted into the hole in the reactor core element to measure the temperature of the fuel rod and to determine the power density of the rod. Even in such an irradiation capsule, when a liquid metal such as Na or NaK is used as the heat transfer medium to be charged into the respective gaps in place of the conventional gas (helium), the power from the nuclear fuel material can be more precisely determined and the maximum service life of the fuel rod can be also more precisely determined than when the gas heat transfer medium is employed. However, sodium has a melting point of 97.8 .degree.C and is in a solid phase at normal temperature. Furthermore, since sodium is an easily oxidized material, when the sodium heat transfer medium is charged into the nuclear fuel rod, it is necessary that the fuel rod and sodium be maintained at a temperature above the melting point of the sodium while the sodium is being charged into the rod. In addition, the charging amount of the sodium should be controlled to a suitable level within the fuel rod. In addition, since the gap defined between the fuel material and sleeve enclosing the material is very narrow, on the order of 0.1 -0.5 mm, when the heat transfer medium is charged into the gap, a void or voids will be easily formed in the sodium heat transfer medium charged into the gap. The formation of a void or voids is undesirable because the void or voids may increase the temperature of the nuclear fuel material to an undesirably high degree (a hot spot or spots are formed in the nuclear fuel rod). Determination of the presence of a void or voids in the heat transfer medium is very important from the standpoint of safety such as the prevention of destruction of the fuel rod and pollution of the primary cooling system during the operation of a nuclear reactor in case of a fuel rod and in case of an irradiation capsule, for assuring safety during an irradiation operation and fulfillment of research purposes through the prevention of damage of the experimental nuclear fuel rod and capsule vessel and of pollution of the primary cooling systen in an experimental nuclear reactor. Thus, it is necessary to inspect the amount (level) of the heat transfer medium charged into the nuclear fuel rod and the presence of a void or voids in the heat transfer medium and if any, the size of the void or voids.
For the inspection of the presence of a void or voids in the above-mentioned sodium-type heat transfer medium, a decisive and satisfactory inspection technology has not yet been found because research and development on the nuclear fuel rods having the sodium-type heat transfer medium themselves are still under way. The X-ray radiography or gamma radiography from an external ray source can scarcely identify a void or voids through the density of a film, since sodium has a relatively low density, the gap is narrow and the object to be inspected is surrounded by a high density material. Flaw detecting inspection by the untrasonic waves is also difficult for the same reason. Thus, under the present state of the art, the quality inspection of nuclear fuel rods has been effected by removing the sleeve from the nuclear fuel material after the sodium-type heat transfer medium has been charged into the nuclear fuel material in a charging procedure considered as effective to prevent a void or voids from forming in the heat transfer medium and observing the presence of a void or voids in the heat transfer medium with the naked eye and if any, the size of such a void or voids and on the assumption based on the data obtained from the naked-eye observation that the same fuel rod production method will provide the same charging level. However, this inspection method is not accurate and is very troublesome.